Silicone rubber sponge exhibits an excellent heat resistance and an excellent weathering resistance, is light weight, and also has a low thermal conductivity and as a consequence is used for automotive parts and components, rolls and belts for image-forming equipment such as copiers, printers, and so forth, and various sealing products. Silicone rubber compositions that incorporate a heat-degradable organic blowing agent or any of various volatile components have heretofore been used to produce silicone rubber sponge, but it has been difficult to use these compositions in mold-based molding and they have a poor capacity to reproduce the shape of a mold cavity.
An addition reaction-curing, silicone rubber sponge-forming composition for fixing roller service is described in JP 2005-062534 A; this composition contains water and a surfactant. However, since it is quite difficult to bring about a uniform and stable dispersion of water in silicone rubber compositions, the described composition is difficult to prepare and the resulting composition also has a poor storage stability.
The silicone rubber sponge-forming composition described in JP 2002-114860 A incorporates a hydrated water-absorptive polymer in a hydrosilylation reaction-crosslinking silicone rubber composition. An emulsion composition for the formation of silicone rubber sponge is described in WO 2005/085357; this emulsion composition comprises a hydrosilylation reaction-crosslinking silicone rubber composition, an aqueous solution of a water-soluble polymer, and an emulsifying agent. These sponge-forming compositions can form a silicone rubber sponge through the formation of a water-containing silicone rubber molding by curing•crosslinking under the application of, for example, heat, followed by removal of the water by the application of, for example, heat, to yield the silicone rubber sponge. However, these sponge-forming compositions have exhibited problems caused by the incorporated polymer, i.e., the appearance of mold staining during molding, a deterioration in the heat resistance of the resulting sponge, and the occurrence of defects in appearance. The fineness of the obtained sponge cells has also been unsatisfactory.
An emulsion composition for the formation of silicone rubber is described in JP 2004-346248 A; this emulsion composition comprises a hydrosilylation reaction-crosslinking silicone rubber composition, water that contains a smectite clay, and an emulsifying agent. However, the sponge moldings obtained with this composition have a large shrinkage rate and this composition also has a poor capacity to reproduce the mold cavity shape; moreover, the fineness and uniformity of the obtained sponge cells are also unsatisfactory.